1. Field of the Art
This invention relates to an optical data code reader which is particularly suitable for use in reading printed data code patterns, for example, for reading bar code labels or the like.
2. Prior Art
As for example of optical data reader for optically reading a printed data code pattern, for example, there have been widely in use the so-called bar code readers for scanning and reading encoded information on bar code labels. The bar code readers of this sort are generally constituted by a light projection means and a reflected light sensor means to read information which is printed on bar code labels in the form of a variable density pattern. The light projection means includes at least a light source and a converging lens to project and converge a light beam toward a bar code label. Further, the reflected light sensor means includes at least a capture lens and a photosensitive element. Reflected light rays from a bar code label are picked up by the capture lens and thereby projected on the photosensitive element to detect variations in density of the printed data code pattern as variations in received light intensity. Therefore, the photosensitive element functions as a photoelectric converter, and output signals of the photosensitive element are fed to a signal processor and processed by known signal processing operations to read and translate the data code pattern.
For reading a printed data code pattern which has a certain length like a bar code, it becomes necessary to scan the data code pattern. For this purpose, it has been the conventional practice to use a line sensor as a photosensitive device, in combination with a light source which is arranged to project light rays over the entire length of a printed data code pattern area by the use of light emitting elements which are arranged in a linear form or by projecting light rays through a slit of a predetermined length. In this connection, it has also been known in the art to employ a movable reflector mirror, for example, a polygon mirror or a galvanomirror for scanning a light beam which is projected from a light source.
Adoption of the above-mentioned arrangements however inevitably results in an optical data code reader which is objectionably large in size and weight. Therefore, compact and light-weight hand scanner type data code readers have been introduced and put into use to replace large and heavy apparatus. The hand scanner type data code readers are adapted to be manually moved along a surface of a printed data code pattern, for example, along a surface of a bar code label.
A hand scanner type optical data reader is disclosed, for example, in Laid-Open Japanese Patent Application H11-39425. This prior art data code reader has, within a pen type casing, a light projection means consisting of a light source and a projected light guide member, along with a reflected light sensor means consisting of a photosensitive element and a reflected light guide member. A ball lens is fitted in a distal end portion of the pen type casing. A light beam which is projected from the light source is converged toward the bar code label, and reflections of the projected light are picked up by the ball lens and shed on the photosensitive element through the reflected light guide member. Therefore, both of projected and reflected light rays are passed through the ball lens. In this instance, the reflected light guide member is constituted by fiber optics, and the light source is constituted by a plural number of light emitting elements which are arranged annularly around a bundle of fiber optics of the reflected light guide member. Further, the projection light guide member is constituted by a collimating lens and a converging lens which has a function of holding fiber optics. A mirror-finish lens tube is located between the converging lens and the ball lens.
The ball lens which is provided at the distal end of the pen type casing is dragged across a bar code label in contact with the surface of the bar code label or in a suitably spaced relation with the latter to scan the bar code, that is to say, to read the data code pattern. For this purpose, the light beam which is projected on the surface of the bar code label needs to be converged to a beam spot of a small diameter at a predetermined position on the bar code label surface. On the other hand, reflected light rays from the bar code label, the light signals indicative of variations in density of the data code pattern, should be securely captured into the optical fibers of the reflection light guide member and transferred to the photosensitive element without losses. Since both projected and reflected light rays are passed through the one and same ball lens, it becomes necessary to provide two separate light paths through the ball lens, i.e., a first light path provided through the center of the ball lens for passage of reflected light and a second annular light path provided around the first light path for passage of projected light. Accordingly, a light flux from the light source as well as a light flux coming out of the projected light guide member is in the form of an annular beam, which is fed to the ball lens as a forwardly converging light flux.
The projected light flux incident on the ball lens is converged to an extremely narrow solid beam pattern at a near point where a bar code label is located. In so doing, it is necessary to prevent the projection light from forming an annular or blurred beam spot on the bar code label surface. In this regard, the mirror-finish tube of the projected light guide member on the side of the light source functions to scatter those light components which would not contribute to converge appropriately on the bar code label.
In the case of the above-mentioned conventional pen type bar code reader in the shape of a hand-operated scanner, both of projected light rays toward a bar code label and reflected light rays from the bar code label are passed through the same ball lens despite extremely large light losses. Namely, in converging an annular light flux, which is projected toward the ball lens, into an extremely narrow solid beam pattern toward the surface of a bar code label, it is necessary to scatter away all of those light components which cannot be converged toward a predetermined position, at the cost of a large amount of light loss. Therefore, there has to be employed a high output type light source, in combination with a photosensitive element of a larger size. Besides, in order to project an annular light flux from a light source and to converge the light flux from the light source to one point, the projected light guide member needs to include a collimating lens and a converging lens between the light source and the ball lens and a space which is enshrouded in a tube, resulting in a projection system which is extremely complicate in construction and large in size to contain a long light path.
On the other hand, on the side of the light receiving system which is so located as to utilize a center portion of the ball lens, it is necessary to position the photosensitive element at least on the rear side of the collimating lens, more specifically, on the rear side of light emitting elements, and to provide a guide means to transfer signal light from a bar code label as far as the photosensitive element. This is the reason why fiber optics are used as a reflected light guide. In such a case, transfer losses are inevitable since the fiber optics have an intrinsic numerical aperture (NA). In addition, noise components are increased by scattered light occurring on the side of the light source as a result of reflections off the tube and ball lens and getting into the fiber optics by reflections. Therefore, in order to prevent these noise component from entering the fiber optics, attempts have been made to limit the maximum acceptance angle by providing a convex surface at a light receiving end of the fiber optics thereby to inhibit entrance of scattered light as much as possible. However, even in that case, a large amount of scattered light still tends to enter the fiber optics to lower the S/N ratio to a considerable degree.
Furthermore, the light projection side is so arranged as to focus a light flux from the light source to a near point to the ball lens. Therefore, even by a slight variation in distance between the ball lens and a bar code label, the beam spot of projected light on the bar code label is blurred or deformed into an annular pattern, failing to converge effectively to a predetermined point. Therefore, in reading a data code pattern of a bar code label by means of a non-contacting type bar code reader, it becomes necessary to control the distance between the bar code reader and the bar code label strictly to avoid reading failures and malfunctioning of the reader although it is extremely difficult to control the movements of the data code reader precisely in reading operations.
In view of the foregoing situations, it is an object of the present invention to provide a data code reader which is reduced in weight and capable of reading data code accurately and reliably from a data code label means even when light is projected from a light source toward a data label medium at a reduced volume.
In accordance with the present invention, for achieving the above-stated objective, there is provided an optical data code reader for reading a data code from a data code label means bearing a printed data code pattern in the form of variations in density, the data code reader comprising: a scanner to be dragged across the data code label means bearing at the time of reading the data code, the scanner being composed of a light projection means including a light source and a converging lens for projecting and converging a light beam toward the label means, and a reflected light sensor means including a capture lens and a photosensitive element for capturing reflected light rays from the data code label means and converting an amount of captured reflection light into an electric signal; the light source being a monochrome light source having a single linear spectrum or a light source of a narrow radiation spectrum range; and the capture lens being constituted by a ball lens and arranged to have a narrow transmission wavelength band permitting passage of a light flux of a source wavelength radiated from said light source while cutting out other wavelengths.
A typical example of the label media to be read by the optical data code reader according to the present invention is the so-called bar code label. However, data can be indicated in a form other than bar codes, for example, by way of a pattern which is printed on paper or on a sheet-like material by the use of characters, figures, designed shapes or the like and which can be optically detected as variations in density. The scanner is manually dragged across the surface of the label means, detecting variations in intensity of reflected light from the label means to read the indicated data code.
In optically reading a data code on a label means, it is preferable to project a beam of monochrome light or of a single linear spectrum toward a label medium, converging the beam to a fine beam spot diameter. The projected light is reflected off the surface of the label means, and the intensity of reflected light varies according to the data code pattern on the label medium. The variations in reflected light intensity are taken into a reflected light sensor means as light signals. At this time, namely, at the time of detecting reflected light, light rays of wavelengths which have not originated from the light source are cut out to enhance the sensitivity and S/N ratio of the reflected light sensor means.
For this purpose, a light beam of a narrow wavelength band width, preferably, of a single linear spectrum is projected from the light source of the scanner. The light source is preferably a monochrome light source, for example, like a laser light source, which include for example a gas laser, a solid laser and a semiconductor laser. In this regard, from the standpoint of making the light source of the scanner compact, it is desirable to employ a semiconductor. Alternatively, among other light sources, it is possible to choose a light source which has a narrow radiation light spectrum range like a light emitting diode.
The light beam which is projected from the light source is converged toward the surface of the label medium by a converging lens which is located in front of and at a predetermined distance from the light source. The converging lens is of a high magnification power having a short back focal length and a longer front focal length. A ball lens is employed as the converging lens because a high magnification power can be obtained from a lens of small size. A ball lens of an extremely small diameter can be produced easily by a polishing operation. It follows that the light projection means including a light source and a converging lens can be constructed in a small and compact form, thanks to the reduction of the distance between light source and the converging lens. Since the converging lens has a long front focal length, the projected light beam can be converged effectively even if the distance between the converging lens and the label means is varied to some extent during a scanning operation. This contributes to make the reading operations or the scanning operations easier. However, in case a ball lens is used as a converging lens, there arises a problem of aberrations, especially spherical aberrations, resulting in blurring of a beam spot at a converging point. In order to suppress spherical aberrations, an aperture is provided on the front side of the ball lens. The aperture size should be large enough for preventing light losses which might result from errors in an assembling stage.
The capture lens of the reflected light sensor means is imparted with a wavelength selectivity in order to separate natural light from signals of reflected light to be shed on the photosensitive element as signals of variations in intensity of reflected light from the label medium. In this regard, if the transmitting wavelength of the capture lens is restricted to exactly correspond to the wavelength of the light beam which is projected on the label medium from the light source, the amount of light which is received by the photosensitive element can be reduced to a considerable degree due to transition of wavelength occurring to light rays which are angularly incident on the capture lens. Therefore, even in a case where the scanner employs a monochrome light source, it is desirable to arrange the capture lens in such a way as to have a selective wavelength band of a certain width including the source wavelength and transition wavelengths, while cutting out wavelengths on the shorter and longer sides of the selective wavelength band. For this purpose, the capture lens is arranged into a narrow wavelength band type with the so-called band pass filter to permit passage only of particular light rays in a narrow wavelength band, providing a reflected light sensor means of simple and compact construction which is virtually constituted by a capture lens and a photosensitive element. Therefore, the data code reader as a whole can be constructed in a small and compact form.
In this connection, in order to let the capture lens function as an optical filter, dichroic layers may be formed on one glass surface by laminating a large number of layers of dielectric material. Tens of dichroic layers are formed on one lens surface by alternately laminating a high refraction film layer H and a low refraction film layer L by a vacuum evaporation process to impart desired spectral transmission characteristics to the capture lens. The dichroic layers can be arranged to have also a function as a band pass filter which permits passage only of light components in a particular wavelength band. However, it becomes necessary to laminate an extremely large number of dichroic layers in order to let them function as a band pass filter. In this regard, the capture lens can be formed of colored glass to absorb or scatter light rays on the longer or shorter side of a selective wavelength band. In this case, the dichroic layers are formed on one side of the colored capture lens thereby to reflect light rays on the other side of the selective wavelength band. As a result, there is obtained a capture lens with a narrow selective transmission band in wavelength, absorbing or scattering light rays of wavelengths on the shorter side of the selective transmission band while reflecting light rays of wavelengths on the loner side of the selective transmission band by the dichroic layers.
As described above, the data code reader of the present invention, which is at least constituted by the light source, converging lens, capture lens and photosensitive element, is simplified in construction and improved to facilitate assembling of its respective parts, particularly precluding the adverse effects of assembling or machining errors as much as possible. Firstly, regarding the ball lens which is employed as a converging lens of the light projection means, what is required in assembling the ball lens is simply to locate the center of the ball lens on an extension line which is drawn from the center of a light emitting point of the light source. There is no need for adjusting inclinations of the optical axis of the converging lens. On the other hand, namely, on the side of the reflected light sensor means, the positions of the capture lens and the photosensitive element can be adjusted in a facilitated manner since there is no need for providing a optical filter for narrowing the transmission wavelength band.
In order to construct the data code reader in a more compact form and at the same time to facilitate the assembling work, the scanner employs a semiconductor laser as a light source of the light projection means, and a photodiode as a photosensitive element of the reflected light sensor means. In this regard, it is desirable to mount the semiconductor laser and the photodiode on a common substrate. Since the light source can be a low output type as mentioned hereinbefore, it is possible for the scanner to employ a small-size semiconductor laser as its light source, which can be provided on a single chip along with a photosensitive element. Further, according to the present invention, the converging lens and the capture lens are mounted on a common support member, which is in turn connected to the above-mentioned common substrate of the semiconductor laser and the photosensitive element to facilitate the assembling of the respective components. More specifically, the converging lens and the capture lens are mounted on a single holder member, which is connected to the substrate. Formed in the holder member is first and second openings for fitting therein the converging lens and the capture lens, respectively. With these arrangements, the converging lens can be set in position simply by abutting same against edge portions around the first opening of the holder member, which also functions as an aperture for the converging lens.
As described above, according to the present invention, the light source and the photosensitive element are integrated into a single chip module by the use of a common substrate, while the converging lens and the capture lens are mounted on a single common holder member. In this case, it is desirable to arrange the optical axes of the light projection means and the reflected light sensor means substantially parallel with each other, and to provide a reflector mirror forward of the converging lens of the light projection means thereby to bend the light path of a projected light beam through a predetermined angle toward the optical axis of the reflected light sensor means. Therefore, reflected light rays off the surface of a label medium are directed angularly toward the capture lens at the same angle as the bending angle of the reflector mirror. In this case, it is possible either to provide a counter reflector mirror on the opposite side, or to let the reflected light rays angularly fall on the capture lens. In this regard, the bending reflector mirror can be omitted in a case where the respective components of the light projection means and reflected light sensor means are set on optical axis which are angularly tilted toward each other in the forward direction.
The output light rays projected from the laser light source have substantially a single linear spectrum. However, depending on the angle of incidence on the capture lens, a shift or transition of wavelength occurs to the light rays. Accordingly, especially in a case where reflected light rays are directed angularly toward the capture lens, it is necessary for the capture lens to have spectral transmission characteristics which permit passage of light rays with a transition wavelength, which occurs to light rays which are incident at an angle smaller than 35 degrees, while cutting out other wavelengths to capture and transfer only reflections of the projected light to the photosensitive element in a secure and reliable manner.
In order to capture reflected light rays from the label medium more effectively, it is desirable to arrange the capture lens into an aspheric form. In this instance, a capture lens of an aspheric shape can be formed, for example, by the use of a lens molding means. Alternatively, the capture lens can be made aspheric by laminating a replica on one surface of a spherical lens. Namely, the capture lens, which is a spherical lens by itself, is made aspheric by laminating a replica on one of its lens surfaces. In a case where dichroic layers are provided on one surface of the capture lens, a replica is provided on the other lens surface. The capture lens is constituted by a planoconvex lens or by a convex lens having spherical surfaces akin to a planoconvex shape, and desirably the dichroic layers are formed on the plane side or almost plane curved surface of the lens and a replica is formed on the other side of the lens to ensure uniformity of the dichroic layers. As for a more specific example of the replica construction, there may be employed a replica of an aspheric shape which is constituted by three concentric zones having curved surfaces of different curvatures, including a center zone occupying a center portion of an effective surface area of the capture lens including an optical axis of the latter and having a curved surface of a smaller radius of curvature than the capture lens surface, an outer zone forming an outer rim of the replica along the outer periphery of the effective surface area of the capture lens and having a curved surface of a larger radius of curvature than the center zone, and an intermediate zone provided between the center and outer zones and having a curved surface of a radius of curvature larger than the outer zone.
The above and other objects, features and advantages of the present invention will become apparent from the following particular description, taken in conjunction with the accompanying drawings which show by way of example preferred embodiments of the invention.